When designing a low-noise amplifier (LNA) it is important to consider its required input matching (e.g. to 50Ω), matching bandwidth, noise figure, linearity and power consumption. If the LNA has a well behaved input impedance the matching network will be easy to design and robust in production. If the resistive component of the LNA's input impedance is very far from the desired matching impedance (e.g. 50Ω) it is very difficult to match it properly without adding extra resistive losses, and, hence, noise. Also a wide-band matching network will be more complex than a more narrow-band one. To keep cost and size down it is important that an LNA can be matched to several input frequencies. This requires wide band LNA structures. Finally, it is normally desirable to have a very high LNA input compression point without sacrificing power consumption.
Two common methods are used for setting the resistive part of the LNA input impedance: resistive shunt or inductive series degeneration.
In the case of a resistive shunt degeneration LNA, illustrated in FIG. 1, the input resistance (Rin) is set by the voltage gain of the circuit and the resistance Rf. Assuming the load is dominated by RL, the transconductance is set by Q1, and the open-loop input impedance is high, then we get Rin≈Rf/(1+gm1·RL), where gm1 is the transconductance of Q1. A typical value for Rf is about 500Ω. The common base transistor Q2 isolates the load resistor from Q1 to increase the voltage gain, and the emitter follower Q3 isolates Rf from RL to minimize loading Such a combination of a common-emitter and a common-base stage is called a cascode, and is a common way to improve the gain and reverse isolation of a single common-emitter stage. Cascodes can be formed of any combination of MOS, BJT and MESFET transistors, including mixed types. A drawback with the resistive shunt degeneration LNA is the presence of the resistor Rf, which degrades the noise figure. One possible way to improve the noise figure is to increase the gain. However, doing so would normally degrade the linearity.
In MOS and MESFET circuits, inductive series degeneration, illustrated in FIG. 2 is more common than resistive shunt degeneration. With the common terminology, the input resistance will be Rin≈gm·Lf/Cgs, where gm and Cgs are the transconductance and the gate-to-source capacitance, respectively, of the transistor M1. A typical value of Lf is 1 nH. The inductive series degeneration is inherently more narrow band as the input impedance basically corresponds to an RLC series resonator, typically with at least a moderate Q (e.g. 1<<Q<10, which can typically be obtained for an on-chip inductor). A drawback with the series degeneration of FIG. 2 is that the LNA closed-loop transconductance is reduced, typically to half the value of M1 alone, and that it is requiring a matching inductor Lg, which typically is external as its value depends on the actual operating frequency. The size of Lf, when integrated on chip, may also be an issue. Furthermore, the structure is inherently narrow band as the input impedance approximately corresponds to a series resonator,
The article Adabi et al, “CMOS Low Noise Amplifier with capacitive feedback matching”, Proc. IEEE 2007 Custom Integrated Circuits Conference, pp. 643-646 (in the following referred to as “Adabi et al”) shows in FIG. 1 therein an LNA with a capacitive feedback.